This invention relates to silencer assemblies for pressurised gases, and has particular, though not exclusive, application to silencer assemblies for use on adsorption gas dryers.
Adsorption dryers for compressed air and gases have been commercially available for many years, and are widely used throughout the world. Although other types of dryers such as deliquescent, membrane and refrigeration dryers, are available, they cannot provide the low pressure dewpoint achieved by adsorption dryers and which is essential for many applications. For instance, a refrigeration dryer can offer a pressure dewpoint of +3 degrees C., whereas an adsorption dryer can give a pressure dewpoint of -70 degrees C.
Normally adsorption dryers employ two containers or towers of desiccant material, commonly known as beds, one of which is `on stream` drying the gas while the other is being regenerated, although single and multi tower systems are also used. In a dual tower dryer, the gas to be dried is passed continuously through one desiccant bed in one direction during the drying cycle, and then, after a predetermined time interval when the bed is considered to have adsorbed sufficient moisture, the inlet gas is switched to the second desiccant bed, and the first desiccant bed is regenerated by heating and/or by evacuation and/or by passing a purge gas therethrough, usually in reverse flow direction.
Adsorption dryers are available in two distinct types, namely heat regenerative and heatless. Heat regenerative, as the name implies, uses heat in one form or another to reactivate the wet desiccant bed, normally in conjunction with a dry purge gas flow from the onstream bed. The heatless dryer uses a purge flow of dry gas, which is usually a proportion of gas from the drying cycle tower and which is passed through the regenerating tower at a lower pressure. The amount or percentage of this dry purge gas is generally directly proportional to the pressure in the regenerating tower.
Furthermore, the regenerating tower pressure is a function of the back pressure created by the desiccant bed, the associated exhaust valve and the associated exhaust silencer as the purge gas strips the moisture from the bed and exits the dryer system.
Hitherto, manufacturers of adsorption dryers have usually incorporated commercially available silencers which, as they become blocked with liberated desiccant dust and moisture, increase the back pressure and hence raise the regeneration tower pressure. For a heatless dryer, this increasing back pressure reduces the purge flow, which, in turn, adversely affects the performance of the dryer unit. For heat-reactivated dryers which utilise external blowers, either a costly positive displacement blower has to be used or, for a fan type blower, the purge gas flow is again reduced which adversely affects performance.
In an effort to overcome these problems, manufacturers have incorporated coarser silencers which, although not blocking as quickly, are very noisy, and are often in contravention of local or national noise regulations. Another way of overcoming the problem is to use two separate exhaust valve systems in which one small valve is used to depressurise the tower through a silencer, after which a larger valve opens to vent the tower to atmosphere. This is a costly solution, in that, not only is another valve system required, but also more complex and costly control systems are required. Furthermore, if the small silencer blocks then there is a chance that the larger valve will open before the tower has depressurised, causing excessive noise and damage to the desiccant.